Working with PDF Annotations in C# and .NET

using System.IO;
using System.IO.Compression;
using SautinSoft.Pdf.Annotations;
using SautinSoft.Pdf;

class Program
{
    /// <summary>
    /// Annotations.
    /// </summary>
    /// <remarks>
    /// Details: https://sautinsoft.com/products/pdf/help/net/developer-guide/attachment-annotations-from-file.php
    /// </remarks>
    static void Main()
    {
        // Before starting this example, please get a free 100-day trial key:
        // https://sautinsoft.com/start-for-free/

        // Apply the key here:
        // PdfDocument.SetLicense("...");

        using (var document = PdfDocument.Load(Path.GetFullPath(@"..\..\..\simple text.pdf")))
        {
            // Extract all the files in the zip archive to a directory on the file system.
            ZipFile.ExtractToDirectory(@"..\..\..\Attachments.zip", "Attachments");

            var page = document.Pages[0];
            int rowCount = 0;
            double spacing = page.CropBox.Width / 5,
                left = spacing,
                bottom = page.CropBox.Height - 200;

            // Add file attachment annotations to the PDF page from all the files extracted from the zip archive.
            foreach (var filePath in Directory.GetFiles("Attachments", "*", SearchOption.AllDirectories))
            {
                var fileAttachmentAnnotation = page.Annotations.AddFileAttachment(left - 10, bottom - 10, filePath);

                // Set a different icon for each file attachment annotation in a row.
                fileAttachmentAnnotation.Appearance.Icon = (PdfFileAttachmentIcon)(rowCount + 1);

                // Set attachment description to the relative path of the file in the zip archive.
                fileAttachmentAnnotation.Description = filePath.Substring(filePath.IndexOf('\\') + 1).Replace('\\', '/');

                // There are, at most, 4 file attachment annotations in a row.
                ++rowCount;
                if (rowCount < 4)
                    left += spacing;
                else
                {
                    rowCount = 0;
                    left = spacing;
                    bottom -= spacing;
                }
            }

            // Delete the directory where zip archive files were extracted to.
            Directory.Delete("Attachments", recursive: true);

            document.Save("File Attachment Annotations from file system.pdf");
        }

        System.Diagnostics.Process.Start(new System.Diagnostics.ProcessStartInfo("File Attachment Annotations from file system.pdf") { UseShellExecute = true });
    }
}

Download